The manufacture of semiconductor apparatus and of various recording media require the application of controlled amounts of liquid to the surface of material in process. It is common practice to dispense liquid to a surface of a wafer or disk which is spinning about it's major axis. The spinning motion causes the liquid to flow evenly over the surface of the disk or wafer. In such applications, uniformity of product requires that the volume of the liquid dispensed and the output flow rate as a function of time be accurately controlled and reproducible.
U.S. Pat. No. 4,690,621, which issued on Sept. 1, 1987, shows a pneumatically operated diaphragm pump which has an integral filter and pneumatically operated valves which are integrated into the pump body.
U.S. Pat. No. 4,483,665, which issued on Nov. 20, 1984 is an example of a bellows type pump which utilizes an external filter, and air under pressure is employed to compress the bellows to discharge liquid from the pump.
As noted earlier herein, the volume dispensed per cycle of pump operation and the rate of discharge as a function of time are important in achieving uniformity of distribution of the liquid to the surface being coated and to uniformity of product.
The use of air or other gases as a driving force, because of their compressibility, does not permit either accurate control of the volume dispensed per cycle or of the dispenser output flow as a function of time.